A well-known driving assistance technology analyzes a driver's face image captured by an in-vehicle camera to determine the condition of a driver that affects driving safety, such as the awareness, aimlessness, and drowsiness of the driver during driving (the condition of the driver is referred to as the “driver condition” in this description), and issues, for example, a warning as needed. For accurate determination of the driver condition, it is preferred that the driving assistance technology be capable of capturing a face image that is as sharp as possible.
Incidentally, vehicles are generally configured so that a seat position can be adjusted in a front-rear direction on the basis of the physical size and preferences of the driver. Adjusting the seat position also moves the position of the face of the driver in the front-rear direction. Under these circumstances, a technology for estimating the face position from the seat position adjusted by the driver and adjusting the focal point of the in-vehicle camera is proposed in order to capture a sharp face image even when the position of the face is moved (Patent Literature 1). Additionally proposed is a technology for capturing a face image in consideration of a change in the posture of the driver during driving (Patent Literature 2).
Meanwhile, a fixed-focus in-vehicle camera having a simple structure is also widely used in place of an in-vehicle camera capable of adjusting the focal point. The fixed-focus in-vehicle camera is not capable of moving the focal point. Therefore, the fixed-focus in-vehicle camera is used with an optical aperture decreased (with the f-number increased) in order to obtain a sharp image even when the face position of the driver is moved forward or rearward. The reason is that a sharp image is obtained without regard to forward or rearward movement of the face position as the depth of field (the range of a subject position within which a focused image is obtained) increases with a decrease in the optical aperture (an increase in the f-number). However, a high-intensity light source is required to compensate for light intensity insufficiency when the aperture is decreased (the f-number is increased). Consequently, setup is performed without excessively decreasing the optical aperture. Ideally, therefore, the optical aperture is set in such a manner that the depth of field agrees with the range of face position movement of the driver.